The invention relates generally to the method and apparatus for generating coherent near-millimeter-wavelength radiation. More particularly, the invention relates to a method and apparatus for continuously modulating the coherent radiation generated by a diffraction radiation generator.
In a diffraction radiation generator, such as orotron, a ribbon electron beam is generated by applying a direct voltage between the cathode and anode of an electron gun. Generally, a negative voltage is applied to the cathode while the anode is operated near ground potential. The electron beam is directed through an open resonator formed by two spaced-apart mirrors to a collector disposed on an opposite side of the open resonator from the electron gun. In the open resonator, the ribbon electron beam passes over the surface of a periodic metal structure, e.g., a reflecting diffraction grating partially covering one of the mirrors, and radiates into a mode of the open resonator. The radiation is fed back onto the ribbon beam and bunches the electrons therein. When the proper conditions of synchronism between the electron beam velocity and the phase velocity of a slow wave propagating along the diffraction grating surface are met, the orotron will radiate coherently at a frequency at or near one of the resonant frequencies of the open resonator. The resonant frequencies of the open resonator are determined by the mirror separation. The electron velocity, in the interaction region where the ribbon beam and the radiation field interact, is determined by the grating-to-cathode voltage. The ribbon beam current is controlled by the anode-to-cathode voltage.
In the past, the cathode has been maintained at a negative voltage of a few thousand volts with respect to ground, and the collector, diffraction grating, and mirrors of the open resonator have been maintained at or near ground potential. When pulsed operation of the orotron was desired, the anode was pulsed with a positive voltage relative to the cathode voltage to generate the ribbon electron beam. In such a high voltage pulsing operation, the pulse frequency was limited by the turn on and turn-off times of the electron gun, and the pulses produced exhibited a "leading edge jitter".
Instead of switching the electron gun on and off at high voltage, the orotron output can be pulsed by continuously operating the electron gun and pulsing the diffraction grating at a low voltage to rapidly initiate or cut off the coherent radiation output of the orotron. The coherent radiation generated within the open resonator of the orotron cuts off rapidly as the grating-to-cathode voltage is increased above a point of maximum power or gain. Thus, the electron gun can be operated continuously, and the orotron output rapidly switched on and off by applying a small pulse, less than ten volts, to the diffraction grating. Such continuous operation of the electron gun also negates the leading-edge jitter caused by thermal effects when the beam current is pulsed.
In addition to low voltage pulse modulation of the orotron output radiation, it would be highly desirable to continuously modulate the amplitude or frequency of the orotron output radiation by a low voltage control signal.